1. Field of the Invention
The present invention is related to the field of devices emitting electromagnetic radiation. More in particular, a semiconductor device that emits radiation at a predetermined wavelength is disclosed. A method of producing such device and applications of such device are also disclosed.
2. Description of the Related Art
Semiconductor devices that can emit non-coherent or coherent electromagnetic radiation are known in the art. A number of publications on semiconductor based electromagnetic radiation emitters deals with Light Emitting Diodes (LEDs) or Microcavity LEDs or Microcavity Lasers or Vertical Cavity Surface Emitting Lasers. Examples of such publications are:
H. De Neve, J. Blondelle, R. Baets, P. Demeester, P. Van Daele, G. Borghs, IEEE Photon. Technol. Lett. 7 287 (1995);
E. F. Schubert, N. E. J. Hunt, R. J. Malik, M. Micovic, D. L. Miller, xe2x80x9cTemperature and Modulation Characteristics of Resonant-Cavity Light-Emitting Diodesxe2x80x9d, Journal of Lightwave Technology, 14 (7), 1721-1729 (1996);
T. Yamauchi and Y. Arakawa, Enhanced and inhibited spontaneous emission in GaAs/AlGaAs vertical microcavity lasers with two kinds of quantum wells. Appl. Phys. Lett. 58 (21), 2339 (1991);
T. J. de Lyon, J. M. Woodall, D. T. McInturff, R. J. S. Bates, J. A. Kash, P. D. Kirchner, and F. Cardone, xe2x80x9cDoping concentration dependence of radiance and optical modulation bandwidth in carbon-doped Ga0.1 In0.49P/GaAs light-emitting diodes grown by gas source molecular beam epitaxyxe2x80x9d Appl. Phys. Lett. 60 (3), 353-355 (1992);
D. G. Deppe, J. C. Campbell, R. Kuchibhotla, T. J. Rogers, B. G. Streetman, xe2x80x9cOptically-coupled mirror-quantum well InGaAs-GaAs light emitting diodexe2x80x9d, Electron. Lett. 26 (20), 1665 (1990);
M. Ettenberg, M. G. Harvey, D. R. Patterson, xe2x80x9cLinear, High-Speed, High-Power Strained Quantum-Well LED""sxe2x80x9d, IEEE Photon. Technol. Lett. 4 (1), 27 (1992);
U.S. Pat. No. 5,089,860 Depew, et al. Feb. 18, 1992, xe2x80x9cQuantum well device with control of spontaneous photon emission, and method of manufacturing samexe2x80x9d;
EP-0550963 of Cho, et al xe2x80x9cAn Improved Light Emitting Diodexe2x80x9d.
The presence of a combination of critical parameters in the fabrication of Vertical Cavity Surface Emitting Lasers (VCSELs) makes such lasers suffer from non-uniformity effects over an epitaxially-grown wafer. Examples of parts of said VCSELs with critical parameter values are the two distributed Bragg Reflectors (DBRs), the cavity thickness and the thickness of the quantum well (i.e. the active region). This problem so far has limited array-production of operational VCSELs to 8*8 arrays. An 8*8 VCSEL array was disclosed in the publication xe2x80x9cFabrication of High-Packaging Density Vertical Cavity Surface-Emitting Laser Arrays Using Selective Oxidationxe2x80x9d IEEE Phot. Techn. Lett. 8, 596 (1996), by Huffaker et al. Furthermore, the high current density needed for efficient operation of lasers (due to the threshold current needed for achieving inversion) limits the simultaneous operation of many laser elements in array applications. In addition, though VCSELs allow high-speed small-signal modulation, VCSELs cannot be used efficiently for high-speed large-signal modulation due to the presence of a threshold current.
The development of Microcavity light-emitting diodes (xcexc cavity LEDs) has created efficient and spectrally-narrow semiconductor light sources other than lasers in general and VCSELs in particular. In contrast to lasers, xcexc cavity LEDs do not suffer from any threshold behavior. State-of-the-art Microcavity LEDs have only one DBR, a wavelength cavity and one or more quantum wells that need to be matched in thickness, making design and production less critical. The absence of a threshold in Microcavity LEDs results in far lower current densities being required for array applications. The increased electrical to optical power efficiency of state-of-the-art xcexc cavity LEDs as compared to conventional LEDs improves the applicability of these xcexccavity LEDs in applications such as optical interconnection systems (in particular as arrays of electromagnetic radiation-emitters) and display applications and systems that are critical on the power budget. State-of-the-art xcexccavity LEDs use one or more quantum wells in the center of the cavity (see FIG. 1) that all have to be identical and matched to the cavity wavelength. Electrons and holes flow from opposite sides into the quantum wells and recombine. Switch-on and switch-off of the xcexccavity LEDs are in essence radiative recombination time limited or RC time constant limited depending on which one is shorter. The use of several quantum wells has proven to be essential to reduce saturation in one quantum well, but moves the carrier localization away from the localization at the anti-node of the standing-wave pattern in the cavity. The use of several quantum wells also slows the response and the switch-on and switch-off of the xcexccavity LEDs.
However, for array production of xcexc cavity LEDs, as for instance for optical interconnects, non-uniformities in the growth of these xcexc cavity LED and the signal modulation speed remain critical issues. In the design of xcexc Cavity LEDs one makes use of a quantum well to ensure carrier recombination at the center of the cavity standing-wave pattern (see H. De Neve, J. Blondelle, R. Baets, P. Demeester, P. Van Daele, and G. Borghs in IEEE Photon. Technol. Lett. 7, 287 (1995)). In the design of other state-of-the-art xcexc cavity LEDs such as disclosed by J. Blondelle, H. De Neve, P. Demeester, P. Van Daele, G. Borghs and R. Baets in El. Lett. 31, 1286 (1995) three quantum wells were used to boost efficiency. In said publication, the efficiency of a xcexc cavity LED is boosted by preventing saturation, at the cost of moving part of the active layer away from the localization at the anti-node of the standing-wave pattern in the cavity making the efficiency enhancement less pronounced. Thickness variations of the quantum well across a sample move the emission wavelength in the different area""s of the sample away from the cavity wavelength which reduces the external efficiency. Furthermore, this device requires the thickness of the quantum well to be exactly matched to the wavelength of the cavity.
Ultra-high speed modulation in semiconductor-based electromagnetic radiation emitting devices in prior art publications so far was disclosed only for VCSELs or inefficient LEDs. T. J. de Lyon, J. M. Woodall, D. T. McInturff, R. J. S. Bates, J. A. Kash, P. D. Kirchner, and F. Cardone, in xe2x80x9cDoping concentration dependence of radiance and optical modulation bandwidth in carbon-doped Ga0.1 In0.49P/GaAs light-emitting diodes grown by gas source molecular beam epitaxyxe2x80x9d Appl. Phys. Lett. 60 (3), 353-355 (1992) disclose a method of making high-speed LEDs by highly doping the active region of the LED, which leads to fast non-radiative recombination and hence a high-speed response of the LED. The resulting gain in speed however is more than compensated by a reduction in quantum efficiency which is incompatible with its use in arrays.
EP-0473983 discloses a light emitting device, the device concept of which uses cavity quantum electrodynamics. This device concept is based in the presence of a quantum well layer adjacent to a barrier layer. The device concept of EP-0473983 suffers a.o. from the problems of:
limited power performance;
slow switch-off time; and
emission wavelength shift during operation.
It is an aim of the present invention to provide a semiconductor-based device emitting electromagnetic radiation, which has a high quantum efficiency, and wherein the precise thickness of the layers composing the device are not critical. The fact that the thicknesses of the layers composing the device are not critical will allow higher yield in growing the structures and higher yield across a wafer. The absence of carrier trapping phenomena in the device according to the invention allows fast charge separation and thus leads to ultra-high-speed, large-signal modulation previously only observed in VCSELs or inefficient LEDs.
The present invention removes the critical thickness of the quantum well (or more quantum wells) of prior art light-emitting devices by replacing it with a bulk layer or a bulk structure, so as to ensure homogenous efficiency over a larger array of devices over a wafer. To ensure no loss in external efficiency, carrier localization at the anti-node of the standing-wave pattern in the cavity is obtained by the addition of a barrier layer with non-critical thickness. This leads to an electromagnetic radiation emitting device with very high external efficiency and with a minimal variation in device characteristics when an array of devices is formed on one semiconductor wafer.
A device for emitting electromagnetic radiation at a predetermined wavelength is disclosed, said device having a cavity comprising a first bulk region of one conductivity type and a second bulk region of opposite conductivity type and wherein a barrier is provided for spatially separating the charge carriers of said first and said second bulk region, said barrier being near the antinode of the standing wave pattern of said cavity, the recombination of the charge carriers of the different conductivity types at/across the barrier creating said radiation. The emission wavelength of said radiation is affected or influenced by said cavity. Said first bulk region is adjacent to and abuts said barrier. Said second bulk region is adjacent to and abuts said barrier. With bulk region, it is meant a region of sufficient thickness for having the quantisation effects on the charge carriers being negligibly and much smaller than the thermal energy (kT) of the charge carriers. Thus, the quantisation effects of a bulk region are not measurable in the emission of the radiation and these effects have no impact compared to the line width of the emitted wavelength. Thus, such bulk region, e.g. is not a quantum well. Said barrier can be a third region in said cavity providing a barrier for transport of said charge carriers in-between said first and said second region, the charge carriers of one conductivity type thereby being trapped at one side of said barrier in either one of said first or second regions, the charge carriers of the other conductivity type being injected from the other side of said barrier, the recombination of the charge carriers of the different conductivity type creating said radiation.
The device can further comprise a mirror being provided on the surface of one of said first or said second region; and a mirror or semi-transparent mirror being provided on the surface of another of said first or said second region. An array of such devices can be made wherein said devices are provided on one substrate. Said first and said second regions can each consist in essentially a first material with a first bandgap, the third region consisting essentially in a third material with a third bandgap.
In an alternative embodiment of the present invention, said first and said second regions consist in essentially a first material and essentially a second material respectively with a first and a second bandgap respectively. The first region and the second region in both embodiments can also comprise layers of a fourth or fifth or further materials.
The cavity can be a single wavelength cavity, a so-called xcex-cavity. The cavity can also be a so-called nxcex-cavity. These terms xcex-cavity and nxcex-cavity are well-known in the art, such a cavity being also well-known in the art.
Further is disclosed a method of producing a device for emitting electromagnetic radiation at a predetermined wavelength, comprising the steps of: depositing a first layer including a first material with a first bandgap and having a refractive index n1 and with charge carriers of a first conductivity type on a substrate; depositing a third layer of a third material with a third bandgap on said first layer, said third bandgap being larger than said first bandgap; and depositing a second layer of substantially the same thickness as said first layer on said third layer, said second layer being provided with charge carriers of a second conductivity type, the total thickness of said first and said second regions having a value of about said predetermined wavelength divided by n1; while maintaining during said deposition steps at least one surface of said first layer and one surface of said second layer essentially parallel. The method can further comprise the step of depositing a mirror layer on said substrate.